Enable public access to SimilarityIndex scoring function

Enable public access to SimilarityIndex scoring function

The SimilarityIndex class implements the useful capability of scoring
the similarity between two files. That capability is required for a
feature that's being developed in another package, to detect files
derived from a set of potential sources.

This CL adds a public factory method to create a SimilarityIndex from
an ObjectLoader. It grants public access to the SimilarityIndex class,
the score method, an inner exception class and a special marker
instance of that exception class.

Change-Id: I3f72670da643be3bb8e261c5af5e9664bcd0401b
Signed-off-by: David Pletcher <dpletcher@google.com>